a. The Field of the Invention
The present invention is generally concerned with the art of road surface traffic regulating signs and markings more particularly with retro reflecting elements for providing night time visibility to horizontally located markings laid or formed on road surfaces, such as traffic lane dividing lines, pedestrian crossings or other signs and markings, as well as lane edge defining lines;
b. The Prior Art
This art is well known and a well worked one and a wide patent literature is available thereon. The forming of and providing sharp night time visibility at distance to traffic lane dividing and edge defining lines is essential for traffic safety. In the specification of the U.S. Pat. No. 3,587,415 granted to the instant applicant (U.K. Pat. No. 1,245,834) the utility and the principles of retro reflective road marking materials have been set forth. An analysis about the optics of retro reflecting elements including each a spherical transparent globule having a mono layer of reflectorized microspheres associated thereto has been disclosed in the applicant's U.S. Pat. No. 3,894,791 (U.K. Pat. No. 1,343,196).
This art has been consistently improved and even more efficient retro-reflective elements comprising globules either of glass or of optically transparent resins have been proposed and devised. The applicant's U.S. Pat. No. 4,072,403 which is hereby incorporated by reference has set forth an improved retro reflecting assembly comprising a not spherically symmetrical transparent resinous globule having a surface sub-divided into a generally part spherical part, a second generally flat part, and a third annular and outwardly convex part between said first and second part, said first part being uncoated and having a predetermined relatively large radius of curvature, said third part having at least one radius of curvature substantially smaller than said large radius, and a monolayer of externally reflective transparent microspheres covering all of said third part and at least a portion of said second part, said microspheres each being only fractionally as large as said globule.
The said globules consist of a transparent polymeric material having a refractive index preferably comprised from about 1.45 to 1.68. This material preferably consists of a polymethylmetacrylate, or a copolymer of methylstyrene and methylmetacrylate, or a copolymer of styrene and acrylonitrile, or a polycarbonate, or also of an epoxy copolymer.
The same U.S. Pat. No. 4,072,403 has also set forth an improved process for manufacturing said asymmetrical globules, said process comprising the steps of forcedly passing a sheet of transparent thermoplastic polymeric material between adjacent counter-rotating calendering rollers having die forming recesses evenly and closely arranged on their cylindrical surfaces to produce upon the action of heat and pressure, an intermediate essentially planar asymmetrical globules article comprising pluralities of essentially part spherical and respectively of rather flat bulges oppositely located at either faces of a plane defined by thin and easily breakable sheet portions which interconnect the asymmetrical globules formed by each pair of oppositely located part spherical and respectively rather flat bulges. This planar asymmetrical globules article for the retro-reflecting use on the road surface is cut into slices or is subject to breaking. In order to separate the individual retro reflecting globules the rather flat bottom is of enormous importance for the best impact resistance to the traffic abuse.
It is evident that the geometry of the bulges and therefore of the globule depends from that of the die forming recesses of which the said calendering rollers had been provided and therefore that no practical limitation exists about the selection of such geometry.
Essentially, the further improved retro reflecting globule has, additionally to one first vertical asymmetry condition relatively to the plane in which the globule in service is secured on the road surface, at least a horizontal asymmetry condition relatively to at least another plane perpendicular to the first mentioned plane.
Particularly, the retro-reflecting globule has a non circular configuration when seen from above in said road surface. More particularly, said non circular configuration is elongated, still more particularly elliptical.
These and other objects and features of the invention will now be made apparent from the following detailed description of preferred embodiments of same invention, taken together with the accompanying drawing;